In a microscope or the like that uses a charged particle beam, scanning with a charged particle beam to irradiate a sample is performed two-dimensionally, that is, in the horizontal direction and the vertical direction, and secondary signals generated from the irradiated region are detected. The microscope amplifies the detected signals and integrates the signals using electric circuits, and then maps the integrated signals to the scan coordinates of the charged particle beam to form a two-dimensional image. At this time, the microscope or the like replaces the amplitudes of the detected signals with gray levels of image data, thereby representing the structure of the sample surface as a two-dimensional image, and displays the image on a monitor such that it is observable. It should be noted that a detected signal acquired with a detector is influenced by the transfer function of an electrical circuit in a process in which the detected signal is converted into image data.
The main factors that affect2009-the transfer function are two: a detector and an amplifier circuit. A detector has a transient response speed from the time a secondary signal is detected until the detected signal is converted into an electrical signal, and such a transient response speed has influence on the gray level of the acquired image data as a transfer function. The transfer function differs from detector to detector, depending on their types, but even detectors of the same model have different transfer functions. Meanwhile, not only an amplifier per se but also an amplifier circuit has frequency characteristics as with the detector, and such frequency characteristics have influence on the gray level of the acquired image data. Such characteristics differ depending on the type and gain value of the amplifier circuit. As described above, a frequency band-limiting filter equivalently exists on a path from the detector to the digital converter and the characteristics of such a filter have a large influence on the analysis of an image.
When a detected signal changes in a shorter time than the response speed of the detector and the amplifier circuit (i.e., a time in which the response converges) on the path, it becomes impossible for the response to follow the change of the detected signal, so that a response in which a next detected signal is convolved with the previous detected signal is output. When such a phenomenon occurs, the output image data is distorted in the scan direction of the charged particle beam, with the result that a blurred image is acquired.
Among the detectors are detectors that use various schemes such as a semiconductor detector, a scintillator, and a microchannel plate. Among them, the scintillator scheme has a higher response than the semiconductor detector, and not only has the device structure thereof been studied but also the circuit thereof has been improved to solve the aforementioned problem. However, none of such methods has led to a definitive resolution or solved the occurrence of the blurring phenomenon.